1. Field of the Invention
The present invention relates to a transfer unit including a plurality of supporting rollers, a transfer belt wound around the supporting rollers to be driven for rotation, and a unit frame rotatably supporting the supporting rollers, and an image forming apparatus including the transfer unit.
2. Description of the Related Art
It has been conventionally known that the transfer unit of the type mentioned above is adopted into an image forming apparatus configured as, for example, an electronic copier, a printer, a facsimile machine, or a Multifunctional Peripheral (MFP) with at least two of these functions. Such an image forming apparatus can be broadly divided into an intermediate transfer type and a direct transfer type. In the intermediate transfer type, a toner image formed on an image carrier is transferred onto a transfer belt of a transfer unit for primary transfer, and the toner image on the transfer belt is then transferred onto a recorded medium for secondary transfer to obtain a recorded image. In the direct transfer type, a toner image formed on an image carrier is directly transferred onto a recording medium carried and conveyed by a transfer belt of a transfer unit to obtain a recorded image (see, for example, Japanese Patent Application Laid-Open No. 2004-151389 and Japanese Patent Application Laid-Open No. 2004-108412).
In the conventional transfer unit, a supporting roller around which a transfer belt is wound includes a cylindrical member and a pair of shaft members press-fitted inside the cylindrical member from an opening of each end in a longitudinal direction of the cylindrical member, and each shaft member is rotatably supported to a unit frame via a bearing. Here, if the center axis line of the shaft members is significantly decentered with respect to the center axis line of the cylindrical member, periodical speed fluctuations occur to the transfer belt wound around the supporting roller and driven for rotation, thereby causing density unevenness in a toner image transferred on a recording medium. In the case of an image forming apparatus that forms a full-color image, a color shift occurs to the toner image on the recording medium. To get around this problem, the shaft members are required to be manufactured so that the center axis line of the shaft members accurately matches with the center axis line of the cylindrical member. Manufacturing cost of such shaft members, however, is high. Therefore, the conventional transfer unit inevitably has a drawback of increasing cost.